Thermally and optical induced effects on the structural and electrical parameters of nc-CdS thin films

Nanocrystalline cadmium sulfide (nc-CdS) thin films have been prepared on well-cleaned glass substrates at room temperature (300 K) by thermal evaporation technique using inert gas condensation method. The effect of light soaking and annealing on the structural and optoelectrical properties of CdS thin films have been studied in this paper. SEM analysis reveals that the grains are irregular in shape and are uniformly distributed over the glass substrate. XRD study indicates an increase in the crystallite size after thermal annealing and a decrease in the crystallite size after light soaking treatment. The current transport mechanisms have been studied as a function of temperature over the temperature range 125-370 K. Above 330 K, thermionic emission contributes to the conduction mechanism. Both thermally assisted tunneling of carriers through and thermionic emission over the grain boundary potential have contributions to the conduction in the temperature range 170-330 K. Below 170 K, Mott's hopping process appears to be the predominant conduction mechanism. The photosensitivity increases after annealing and decreases after light soaking. Transient photoconductivity measurements indicate that the decay of photoconductivity is slow and non-exponential indicating the presence of continuous distribution of defect states in the energy gap.